Specification guide · Dubbo & the Orana

Which shed steel and frame grade actually suits a Dubbo block

For most blocks around Dubbo the right answer is a cold-formed C-section frame in G550 steel, Z275 galvanised purlins and girts, 0.42mm BMT Colorbond or Zincalume cladding and Class 3 screws, with whatever is left over going into deeper footings rather than fancier steel. On a 12m x 9m shed that specification sits roughly $1,500 to $4,000 above the cheapest kit, and it is the difference between a building that is still square in 25 years and one that racks in the first bad wet-dry cycle. Move up to a hot-rolled portal frame once you pass about a 15m clear span.

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The comparison

Specification by specification, what it costs and what it buys

Every line below is a decision someone will ask you to make, or quietly make for you. The costs are 2026 figures for the Dubbo and Orana region on a shed of roughly 100 to 200 square metres.

Option	Typical cost here	Life in Dubbo conditions	Maintenance load	Warranty position	Who it suits
Cold-formed C-section frame, G550, Z275	Standard inclusion	30 years plus	Bolt check every few years	Builder workmanship terms plus the steel maker material warranty	Spans to about 15m: nearly every garage, barn and farm shed out here
Hot-rolled portal frame, universal beams	+15% to 30% on the structure, often $10,000 to $35,000	40 years plus	Repaint exposed sections at 15 to 20 years unless hot dip galvanised	Same, plus the fabricator certificate	Clear spans over 15m, gantry cranes, mezzanines, commercial buildings
Zincalume AZ150 cladding, unpainted	Cheapest sheet, about $6 to $12 per m² under Colorbond	Similar to Colorbond this far inland	Hose the sheltered areas; keep it away from fertiliser and wet concrete	Steel maker product warranty, conditional on install and washing	Machinery sheds, hay sheds, anything where looks do not matter
Colorbond 0.42mm BMT	+$1,000 to $2,500 on a 12m x 9m shed	30 years plus in this low corrosivity air	Wash the unwashed areas once or twice a year	Product warranty, conditional and worth reading	Garages, barns, sheds visible from the house or the road
0.48mm BMT sheet, either finish	+$1,200 to $4,000	Same corrosion life, far better against dents and hail	No change	No change	Exposed cleared paddocks, hail worry, walls at machine height, wide purlin centres
Class 3 screws to AS 3566	+$150 to $500 over the cheap fixing	Matched to the sheet	None	A sheet warranty can be void if the fixing class is wrong	Every shed in the Orana. Class 4 is a coastal spec you do not need
Footings sized to an AS 2870 classification	+$1,500 to $8,000	Decides whether the frame is still square in 30 years	Keep water draining away from the pad	An engineering item, not a warranty item	Every block on grey or black cracking clay, which is most of the flats
Roof insulation blanket, 55mm or 75mm	$18 to $32 per m² installed	20 years plus if the facing stays intact	Keep birds and mice out of the facing	Manufacturer warranty on the product only	Workshops, stables, anything you stand inside on a 40°C January day

Indicative 2026 figures for the Dubbo and Orana region. For whole-building bands see the pricing guide and the shed cost in Dubbo page.

Read the label

What the numbers on the steel actually mean

There are three different numbers on a shed quote and people mix them up constantly, which is how two quotes that look identical can be thousands of dollars apart in what you get.

G550 and G450 are strength. They are the yield strength of the steel in megapascals, so G550 steel yields at 550 MPa. Cold-formed shed sections are usually rolled from G550 in the thin gauges and G450 in the heavier ones. A higher grade lets the roll former use less steel for the same load, which is why a modern G550 frame can be lighter than the shed your grandfather put up and still stronger. It says nothing at all about rust.

Z275, AZ150 and AM125 are corrosion protection. These are coating classes under AS 1397, and the number is the mass of coating in grams per square metre counting both faces. Z275 is 275 grams of zinc, the normal specification for purlins, girts and structural sections. AZ150 is 150 grams of a 55 per cent aluminium zinc alloy, which is the Zincalume coating under cladding sheet and outperforms plain zinc for the same coating mass. AM125 is a newer zinc aluminium magnesium coating. Lighter classes exist, and a Z100 or Z200 purlin in a shed on ground where water pools is a false economy you will smell before you see.

BMT and TCT are thickness. BMT is base metal thickness, the steel on its own. TCT is total coated thickness, the steel plus the metallic coating plus the paint. A 0.42mm BMT sheet is commonly quoted as 0.47 TCT, so a quote written in TCT reads about 12 per cent thicker than the identical sheet written in BMT. Ask every quote to state BMT and compare like with like. This is the single most common way a cheap shed hides being a cheap shed.

Local conditions

Three Dubbo conditions that decide the specification

Corrosion is not your main enemy out here

Dubbo sits roughly 350km from the ocean on the western slopes and plains. Under the atmospheric corrosivity categories in AS 4312, inland country like this is a low corrosivity environment, a long way from the severe marine categories that drive coastal specifications. That has a real dollar consequence: the premium coastal cladding and Class 4 fixings that a shed at Coffs Harbour genuinely needs are money you can put somewhere more useful here.

The corrosion that does bite in the Orana is chemical and local, not atmospheric. Urea and superphosphate stored hard against a wall will eat a sheet from the inside in a handful of years. Cattle and sheep effluent under a shelter does the same. Bore water with high salts used for washdown is harder on steel than rain ever is. Fresh concrete splashed onto unpainted Zincalume during the pour leaves a permanent stain and a corrosion cell. If your shed will do any of that, say so at quote stage: the specification then changes in that zone only, which is far cheaper than upgrading the whole building. Our Colorbond versus Zincalume comparison goes deeper on the finish choice.

Reactive clay is your main enemy

The grey and black cracking clays through the Macquarie Valley are highly reactive soils. They swell when wet and shrink when dry, and the movement is not subtle. Under AS 2870 a site is classified from Class A through S, M, H1, H2 and E, where the class reflects characteristic surface movement: roughly 20 to 40mm on Class M, 40 to 60mm on H1, 60 to 75mm on H2 and beyond 75mm on Class E. Plenty of blocks on the flats around Dubbo, Narromine and Geurie land in H1 or worse.

A shed footing that lifts 50mm under one column and not the next racks the frame, jams the roller door and splits the sheet at the fixings. No cladding upgrade saves you from that. It is why the honest answer to "which steel should I buy" is often "spend it under the ground instead". A site classification and properly designed pad footings typically add $1,500 to $8,000, and they are the best value line on the whole quote.

Region A wind, but not suburban Region A

Dubbo and the Orana are in Region A under AS/NZS 1170.2, the standard inland wind region, with a regional gust speed for the 1 in 500 year event of around 45 metres per second before any site factors are applied. The catch is terrain category. A sheltered block in town with houses and trees upwind is a very different exposure to a cleared cropping paddock toward Narromine or Gilgandra, where the wind arrives with nothing in front of it. The same shed on the same day can need noticeably heavier portals, closer purlin centres and more roof bracing on the paddock than it does in town.

Open-front sheds are the special case. Wind that gets inside an open bay pressurises the building and pushes the roof up while the outside is sucking it up, so an open-front machinery or hay shed carries far more uplift than the same building closed in. That is a frame, bracing and tie-down question, not a cladding question, and it is the one people underestimate most.

The cheap specification that fails early around Dubbo

Nobody sells a bad shed on purpose. What happens is a series of small, invisible reductions that each save a few hundred dollars. Here is what they look like in the field, roughly in the order we see them fail.

A nominal footing. A 600mm hole because that is what the generic kit drawing said, on ground that moves 60mm a season. The first symptom is a door that will not latch in February.
Light purlins at wide centres. 1.0mm purlins at 1,500mm centres will hold a roof up on a calm day. In a westerly gust front they let the sheet flex, the screw holes elongate, and then the holes leak.
A kit engineered for the wrong terrain. A certificate that assumes a sheltered suburban block, then the shed goes up on 40 cleared acres. The paperwork is real, it just describes a different site.
Class 2 fixings. A few hundred dollars saved across the whole building, and rust weeping down a wall inside five years. The sheet warranty generally will not help you, because the wrong fastener class is an installation fault.
Short sheets and no overhang. A 30mm overhang instead of 50mm means water tracks back under the sheet into the top of the wall. Cheap to get right, expensive to chase later.
Bracing left out where the engineer wanted it. The building still stands. It also racks a few millimetres in every big blow, until the doors stop tracking and the sheets show diagonal creases.

Every one of those is invisible from the driveway on handover day, which is exactly why the specification and not the sales pitch is what you compare. If a shed you already own is showing some of these symptoms, our shed warning signs page walks through what is urgent and what can wait until autumn.

Want the specification written down before you compare quotes?

We put the frame section, gauge, coating class, sheet BMT, screw class and site classification on the quote in writing, so you can hold every other quote up against it.

Get a written quote Work out the size first

What "commercial grade" and "heavy duty" are actually worth

Nothing, on their own. There is no Australian Standard for "commercial grade", "farm tough", "premium" or "heavy duty". They are adjectives, and every shed sold in this country has some of them on the brochure. The moment you ask for the number behind the adjective one of two things happens: you get the number, which is useful, or you get another adjective, which is also useful, because now you know.

Here are the eight numbers that turn a sales pitch into a comparable specification. Ask for all eight in writing and every quote in your pile becomes the same document with different prices on it.

Frame type and section size: cold-formed C-section or hot-rolled universal beam, and the actual section, for example C25024 or 250UB.
Purlin and girt gauge and centres, in millimetres.
Coating class on the structural steel: Z275, or something lighter.
Cladding profile and thickness stated as BMT, not TCT.
Fastener class to AS 3566, and whether the same class is used on the roof, the walls and the flashings.
The wind region, terrain category and importance level used in the design, and who signed the design certificate.
The AS 2870 site classification, who did it, and the footing size that came out of it.
What is excluded: slab, doors, gutters, tie-down bolts, delivery to a farm gate, crane hire, council certification.

That last line is where most of the price difference between two quotes actually lives. Before you sign anything, check what approval your build needs on the rural NSW shed permits page, and read the questions on how to choose a shed builder.

Our straight recommendation for a typical Dubbo build

Town block garage or workshop, up to about a 9m span. Cold-formed C-section frame in G550 with Z275 sections, 0.42mm BMT Colorbond roof and walls, Class 3 screws, a 100mm slab designed to the site classification, and a proper 50mm overhang with gutters sized for the roof area. Skip the 0.48mm sheet unless there are trees dropping limbs on it. Put the saved money into the door instead: a wider, taller roller door is the upgrade you will feel every week for 20 years. The garages page has the sizes that work.

Rural machinery or hay shed on cleared ground. Same frame family, but designed to the real terrain category rather than a generic rating, with heavier purlins at closer centres, full roof bracing, 0.48mm BMT roof sheet, and unpainted Zincalume walls if the building is purely functional. Pad footings sized off a site classification, and the pad built up so water runs away from the building instead of sitting under the columns. A compacted gravel floor is completely acceptable for hay and often better than concrete. Detail on those buildings is on the farm sheds page.

When not to spend the money

We talk people out of three upgrades most weeks. You do not need Class 4 fixings or a premium coastal cladding grade 350km inland unless fertiliser, effluent or bore water is in the picture. You do not need a hot-rolled portal frame under about a 15m clear span, because a properly engineered cold-formed frame does that job for less money. And you do not need an insulated roof on a shed you only walk into to grab a spanner: put that $18 to $32 per square metre into eave height instead, because height is the one thing you can never add later. If the shed you already have is basically sound, a repair and a repaint following the maintenance guide beats a new building every time.

Get a price

Get the steel specification priced for your block

Send us the block and what the shed has to do, and we will come out, check the site and put the frame, coating, cladding and fixing specification in writing, usually within a few days of the visit.

Specification questions we get asked

What steel grade should a shed frame be in Dubbo?

Most shed frames around Dubbo are cold-formed C-sections rolled from G550 steel, which is a 550 MPa yield grade, with a Z275 zinc coating that puts 275 grams of zinc on every square metre of surface. That combination is the sensible default for clear spans up to about 15 metres. Past 15 metres, or where you want a gantry crane or a mezzanine, a hot-rolled portal frame in universal beams is the better structure and adds 15 to 30 per cent to the frame cost.

Is Colorbond worth the extra over Zincalume in the Orana?

Inland at Dubbo the corrosion difference between the two is small, because the atmosphere here is a low corrosivity environment rather than a coastal one. The choice is mostly about looks and heat. Colorbond costs roughly 6 to 12 dollars per square metre more on the sheet, which is about 1,000 to 2,500 dollars on a 12 by 9 metre shed, so use it where the building is visible or lived in and Zincalume where it is a working machinery or hay shed.

What thickness should shed roof sheeting be here?

0.42 mm base metal thickness is the standard and it is adequate for most Dubbo sheds. Step up to 0.48 mm where the site is an exposed cleared paddock, where the purlins sit at wide centres, or where hail and stock damage are a real risk. That upgrade usually adds 15 to 20 per cent to the sheet cost, commonly 1,200 to 4,000 dollars on a farm shed.

What does BMT mean on a shed quote?

BMT is base metal thickness, the steel itself with no coating or paint counted in. TCT is total coated thickness, which includes the metallic coating and the paint film and is always the bigger number. A 0.42 mm BMT sheet is often advertised as 0.47 TCT, so compare BMT against BMT or you are comparing two different things and the thinner sheet looks thicker.

Do I need Class 4 screws or marine grade fixings in Dubbo?

No. Dubbo sits roughly 350 kilometres from the coast in a low corrosivity environment, so Class 3 screws to AS 3566 are the correct specification and Class 4 is a coastal answer to a problem you do not have here. The exceptions are sheds that store fertiliser, sheds over stock effluent and anything washed down with salty bore water, where both the fixing class and the sheet coating need lifting.

How deep do shed footings need to be in Dubbo clay?

There is no single depth, and that is exactly the point. The footing is sized from an AS 2870 site classification, and much of the Macquarie floodplain around Dubbo classifies as H1, H2 or E reactive clay, with 40 to more than 75 millimetres of surface movement between a wet winter and a dry summer. On that ground a generic 600 millimetre kit footing is not enough, and sizing it properly usually adds 1,500 to 8,000 dollars to the job.

Get the right specification for your block, in writing

Free measure, the site checked properly, and a fixed written quote with every gauge and grade spelled out.

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